Arterial blood pressure (BP) exerts a critical influence on physiological homeostasis in humans, and abnormalities in BP regulation have potential pathophysiological consequences. The autonomic nervous system (ANS) is a key regulator of the cardiovascular determinants of BP. There may be age-related changes in ANS support of arterial blood pressure, and these may be at least partially related to differences in adrenergic receptor sensitivity. The general aims of this study are to test the hypotheses that: 1) basal ANS tone makes a significant contribution to the maintenance of resting BP and its circulatory determinants in the healthy adult human; 2) ANS support of BP is increased with primary (healthy) aging; and 3) adrenergic receptor sensitivity is reduced with primary aging. To test these hypotheses we are using the autonomic ganglionic antagonist, Trimethaphan, to eliminate ANS input to cardiovascular tissues, and thereby quantify its contribution to the maintenance of BP. The adrenergic agonists phenylephrine (a-agonist), and isoproterenol (b-agonist) are infused during measurements of heart rate, BP, and calf blood flow, to determine adrenergic receptor sensitivity (i.e., effector tissue responsiveness to adrenergic stimuli). Two groups of healthy subjects are being studied: 27 young healthy (50% female), and 27 older healthy (50% female) adults.